1. Field of the Invention
The present invention relates to a semiconductor laser device to be used for optical disks, optical communications, and the like, and a method of manufacturing such a device. Particularly, the present invention relates to a semiconductor laser provided with a quantum well active layer which device has good characteristics and high reliability.
2. Description of the Related Art
In recent years, to improve speed in writing information to optical disks typified by CD-R/RW and DVD-R/RW, development of a semiconductor laser having an output power of as high as 100 mW has been expected.
To realize a semiconductor laser having a high power and reliability, it is necessary to prevent deterioration of end faces thereof and operate it at low electric current. To do so, it is more advantageous to provide the semiconductor laser device with a structure having a quantum well active layer than with a structure having a bulk active layer because the former is superior to the latter in the gain characteristics thereof.
FIG. 7 is a sectional view showing the structure of a conventional semiconductor laser device having a quantum well active layer. The device is formed as follows. On an n-type substrate 701 are formed a Si doped n-type buffer layer 702, a Si doped n-type cladding layer 703, an undoped optical guide layer 704, an undoped quantum well active layer 705, an undoped optical guide layer 706, a Zn doped p-type cladding layer 707, and a Zn doped p-type cap layer 708. Thereafter, the Zn doped p-type cap layer 708 and the Zn doped p-type cladding layer 707 are processed into the shape of a striped ridge, and a Si doped p-type block layer 709 is formed such that the striped ridge is embedded in the block layer 709. Further, a Zn doped p-type contact layer 710 is formed. In this manner, the semiconductor laser device is constructed. Reference numeral 711 denotes an optical distribution in an optical waveguide.
The optical guide layers 704 and 706 are formed at the lower and upper sides of the quantum well active layer 705 to entrap light in the quantum well active layer 705 and prevent diffusion of impurities into the quantum well active layer 705 from the p-type cladding layer 707 and the n-type cladding layer 703.
In the case of the semiconductor laser device having the bulk active layer, the diffusion of impurities into the active layer causes formation of a crystal defect acting as the center of recombination of optical carriers in the active layer. Consequently, the characteristic of the semiconductor laser device deteriorates. Further, the impurities diffuse easily into the active layer during the operation of the semiconductor laser device. Consequently, the life thereof deteriorates. In particular, Zn that is used as a p-type doping material diffuses at a high speed in a film. As a result, frequently, the active layer has a p-type electrical conductivity and causes a remote junction. In order to solve such a problem, trials of preventing the diffusion of the Zn have been made by forming, between the active layer and the p-type cladding layer, an undoped layer, a layer having an opposite (n-type) electrical conductivity, or a layer having a different composition in which the diffusion speed is low.
On the other hand, in the semiconductor laser device having the active layer composed of quantum well, the undoped optical guide layers formed at both sides of the quantum well layer serve to prevent the impurities from diffusing into the quantum well layer besides its essential roll.
The present inventors have confirmed that in the semiconductor laser device having the active layer composed of the quantum well, the dopant, or impurities diffuse even into the optical guide layer, which is provided to prevent the diffusion of the dopant into the quantum well layer, and that this results in deterioration of the characteristic of the semiconductor laser device. In particular, it has been found that with the diffusion of the dopant at a carrier concentration of more than 5xc3x971017 cmxe2x88x923 from the cladding layer into the optical guide layer, threshold current rises and the reliability of the device deteriorates. The phenomenon occurs for the following reason: In the case where the active layer is composed of the quantum well as shown in FIG. 7, the optical distribution 711 in the optical waveguide is large in the optical guide layers. Therefore, the defect formed by the dopant that has diffused in the optical guide layer acts as the center of recombination of optical carriers that have been distributed in the optical guide layer. The present inventors have also found that the amount of the diffusion of the dopant into the optical guide layer from the cladding layer and the diffusion length thereof depend on the concentration of the dopant of the cladding layer and a manufacturing condition.
It is therefore an object of the present invention to provide a semiconductor laser device with an quantum well active layer which is allowed to offer a high power and a preferable reliability, by preventing impurities from diffusing into an undoped optical guide layer. It is also an object of the present invention to provide a method of manufacturing such a semiconductor laser device.
To solve the above problem, according to an aspect of the present invention, in a semiconductor laser device having a quantum well active layer disposed between a pair of cladding layers, and an optical guide layer disposed between at least one of the cladding layers and the quantum well active layer, an undoped thin spacer layer is provided between the optical guide layer and the at least one of the cladding layers so that the spacer layer absorbs impurities diffusing thereinto from the cladding layer to thereby prevent them from diffusing into the optical guide layer. Therefore, a semiconductor laser device having high reliability can be obtained.
If the thickness of the spacer layer is smaller than 5 nm, the diffusion of impurities, or dopant into the optical guide layer will not be able to be sufficiently prevented. Consequently, the characteristic and reliability of the semiconductor laser device will deteriorate. On the other hand, if the thickness of the spacer layer is 10 nm or larger, an carrier concentration will become low. As a result, an electronic barrier will also be lowered. Consequently, the temperature characteristic of the semiconductor laser device will deteriorate. Accordingly, in order to securely obtain a semiconductor laser device having preferable characteristics and high reliability, the spacer layer may preferably have a thickness of 5 nm or more but less than 10 cm.
If the carrier concentration of a p-type cladding layer is higher than 5xc3x971018 cmxe2x88x923, a large quantity of impurities will diffuse into the optical guide layer. Consequently, the characteristic of the semiconductor laser device deteriorates. On the other hand, if the carrier concentration of the p-type cladding layer is lower than 8xc3x971017 cmxe2x88x923 disadvantageously, the temperature characteristic of the semiconductor laser device will be lowered and an operational voltage will increase. Therefore, it is preferable that the p-type cladding layer has a carrier concentration in a range of from 8xc3x971017 cmxe2x88x923 to 5xc3x971018 cmxe2x88x923.
In an embodiment, the spacer layer has a p-type electrical conductivity, and a carrier concentration at an interface between the spacer layer and the optical guide layer is between 5xc3x971018 cmxe2x88x923 and 5xc3x971017 cmxe2x88x923.
If the spacer layer has an n-type electrical conductivity, this tends to result in formation of a remote junction. If the carrier concentration at the interface between the spacer layer and the optical guide layer is less than 5xc3x971016 cmxe2x88x923, the temperature characteristic of the semiconductor laser device tends to be lowered and the operational voltage will increase. If the dopant diffuses into the optical guide layer at a carrier concentration more than 5xc3x971017 cmxe2x88x923, the device characteristics and reliability of the semiconductor laser device will deteriorate.
The spacer layer may have a composition identical to that of the p-type cladding layer or may be larger than the p-type cladding layer in a band gap.
In this case, light is well entrapped or confined in the active layer. Thus, good device characteristics and good optical emission characteristic are realized
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor laser device, comprising the steps of sequentially forming, on an n-type substrate, an n-type doped buffer layer, an n-type doped cladding layer, a first undoped optical guide layer, an undoped quantum well active layer, a second undoped optical guide layer, p-type doped cladding layer, and a p-type doped cap layer by a vapor phase growth method, characterized by:
forming an undoped spacer layer between the second undoped optical guide layer and the p-type doped cladding layer.
Because the spacer layer is undoped, it is possible to securely obtain a p-type electrical conductivity and prevent diffusion of impurities into the optical guide layer. Also, the use of a vapor phase growth method enables the control of the thickness of the spacer layer in a nm order. Preferably, the undoped spacer layer may be formed in a thickness of 5 nm or more but below 10 nm.
In an embodiment, each of the layers is formed by a MOCVD method and under a condition in which a growth temperature is from 650xc2x0 C. to 800xc2x0 C. both inclusive, and a ratio of a feed rate of a group V source to that of a group III source is from 50 to 200 both inclusive. In this case, it is possible to control the amount of diffusion of the impurities into the optical guide layer from the cladding layer and the diffusion length thereof and secure a good crystallinity. Therefore, it is possible to obtain a semiconductor laser device having good characteristics and high reliability.
The effect and function of the present invention will be described below.
The diffusion of impurities into the active layer of a semiconductor laser device causes formation of a crystal defect acting as the center of recombination of optical carriers in the active layer. Consequently, the characteristic of the semiconductor laser device deteriorates. Further, the impurities diffuse easily into the active layer during the operation of the semiconductor laser device. Consequently, the life of the device decreases. In particular, Zn that is used as a p-type doping material diffuses at a high speed in a film. As a result, the active layer has a p-type electrical conductivity, which results in formation of a remote junction.
In the semiconductor laser device having the quantum well as the active layer, an undoped optical guide layer formed at both sides of the quantum well layer has a roll of preventing the impurities from diffusing into the quantum well layer, in addition to its primary roll. However, as a result of the present inventors"" research, it has been found that when the impurities or dopant diffuses from the cladding layer into the optical guide layer at a carrier concentration of more than 5xc3x971017 cmxe2x88x923, threshold current rises and the reliability of the semiconductor laser device deteriorates. The phenomenon can be considered to occur for the following reason: In the case where the active layer has a quantum well structure and is very thin, light in an optical waveguide is distributed in a comparatively large quantity in the optical guide layer. Therefore, a defect formed by the dopant that has diffused in the optical guide layer acts as the center of recombination of optical carriers. Again, it has been found that the semiconductor laser device whose active layer has a quantum well structure has a deteriorated characteristic when a: dopant such as, for example, Zn diffuses even into the undoped optical guide layer as well as the active layer. According to the present invention, the problem is solved by forming the undoped thin spacer layer between the optical guide layer and the cladding layer so that the spacer layer absorbs impurities diffusing thereto from the cladding layer to thereby prevent them from diffusing further into the optical guide layer. Therefore, according to the present invention, a semiconductor laser device having good characteristics and high reliability was obtained.
The amount of the diffusion of the dopant into the optical guide layer from the cladding layer and the diffusion length thereof depend on a concentration of the dopant of the cladding layer and a manufacturing condition. Therefore, it is possible to produce a semiconductor laser device having a high power and high reliability by optimizing the architecture and the manufacturing or producing conditions regarding the doping concentration and the thickness of each layer.